Potassium channel activators differentially modulate the effect of sodium channel blockade on cardiac conduction

Exploring the Pathophysiology of ATP-Dependent Potassium Channels in Insulin Resistance

Ionic channels are present in eucaryotic plasma and intracellular membranes. They coordinate and control several functions. Potassium channels belong to the most diverse family of ionic channels that includes ATP-dependent potassium (KATP) channels in the potassium rectifier channel subfamily. These channels were initially described in heart muscle and then in other tissues such as pancreatic, skeletal muscle, brain, and vascular and non-vascular smooth muscle tissues. In pancreatic beta cells, KATP channels are primarily responsible for maintaining the membrane potential and for depolarization-mediated insulin release, and their decreased density and activity may be related to insulin resistance. KATP channels' relationship with insulin resistance is beginning to be explored in extra-pancreatic beta tissues like the skeletal muscle, where KATP channels are involved in insulin-dependent glucose recapture and their activation may lead to insulin resistance. In adipose tissues, KATP channels containing Kir6.2 protein subunits could be related to the increase in free fatty acids and insulin resistance; therefore, pathological processes that promote prolonged adipocyte KATP channel inhibition might lead to obesity due to insulin resistance. In the central nervous system, KATP channel activation can regulate peripheric glycemia and lead to brain insulin resistance, an early peripheral alteration that can lead to the development of pathologies such as obesity and Type 2 Diabetes Mellitus (T2DM). In this review, we aim to discuss the characteristics of KATP channels, their relationship with clinical disorders, and their mechanisms and potential associations with peripheral and central insulin resistance.

Extracellular Perinexal Separation Is a Principal Determinant of Cardiac Conduction

BACKGROUND

Cardiac conduction is understood to occur through gap junctions. Recent evidence supports ephaptic coupling as another mechanism of electrical communication in the heart. Conduction via gap junctions predicts a direct relationship between conduction velocity (CV) and bulk extracellular resistance. By contrast, ephaptic theory is premised on the existence of a biphasic relationship between CV and the volume of specialized extracellular clefts within intercalated discs such as the perinexus. Our objective was to determine the relationship between ventricular CV and structural changes to micro- and nanoscale extracellular spaces.

METHODS

Conduction and Cx43 (connexin43) protein expression were quantified from optically mapped guinea pig whole-heart preparations perfused with the osmotic agents albumin, mannitol, dextran 70 kDa, or dextran 2 MDa. Peak sodium current was quantified in isolated guinea pig ventricular myocytes. Extracellular resistance was quantified by impedance spectroscopy. Intercellular communication was assessed in a heterologous expression system with fluorescence recovery after photobleaching. Perinexal width was quantified from transmission electron micrographs.

RESULTS

CV primarily in the transverse direction of propagation was significantly reduced by mannitol and increased by albumin and both dextrans. The combination of albumin and dextran 70 kDa decreased CV relative to albumin alone. Extracellular resistance was reduced by mannitol, unchanged by albumin, and increased by both dextrans. Cx43 expression and conductance and peak sodium currents were not significantly altered by the osmotic agents. In response to osmotic agents, perinexal width, in order of narrowest to widest, was albumin with dextran 70 kDa; albumin or dextran 2 MDa; dextran 70 kDa or no osmotic agent, and mannitol. When compared in the same order, CV was biphasically related to perinexal width.

CONCLUSIONS

Cardiac conduction does not correlate with extracellular resistance but is biphasically related to perinexal separation, providing evidence that the relationship between CV and extracellular volume is determined by ephaptic mechanisms under conditions of normal gap junctional coupling.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Potassium channels in the Cx43 gap junction perinexus modulate ephaptic coupling: an experimental and modeling study

It was recently demonstrated that cardiac sodium channels (Nav1.5) localized at the perinexus, an intercalated disc (ID) nanodomain associated with gap junctions (GJ), may contribute to electrical coupling between cardiac myocytes via an ephaptic mechanism. Impairment of ephaptic coupling by acute interstitial edema (AIE)-induced swelling of the perinexus was associated with arrhythmogenic, anisotropic conduction slowing. Given that Kir2.1 has also recently been reported to localize at intercalated discs, we hypothesized that Kir2.1 channels may reside within the perinexus and that inhibiting them may mitigate arrhythmogenic conduction slowing observed during AIE. Using gated stimulated emission depletion (gSTED) and stochastic optical reconstruction microscopy (STORM) super-resolution microscopy, we indeed find that a significant proportion of Kir2.1 channels resides within the perinexus. Moreover, whereas Nav1.5 inhibition during AIE exacerbated arrhythmogenic conduction slowing, inhibiting Kir2.1 channels during AIE preferentially increased transverse conduction velocity-decreasing anisotropy and ameliorating arrhythmia risk compared to AIE alone. Comparison of our results with a nanodomain computer model identified enrichment of both Nav1.5 and Kir2.1 at intercalated discs as key factors underlying the experimental observations. We demonstrate that Kir2.1 channels are localized within the perinexus alongside Nav1.5 channels. Further, targeting Kir2.1 modulates intercellular coupling between cardiac myocytes, anisotropy of conduction, and arrhythmia propensity in a manner consistent with a role for ephaptic coupling in cardiac conduction. For over half a century, electrical excitation in the heart has been thought to occur exclusively via gap junction-mediated ionic current flow between cells. Further, excitation was thought to depend almost exclusively on sodium channels with potassium channels being involved mainly in returning the cell to rest. Here, we demonstrate that sodium and potassium channels co-reside within nanoscale domains at cell-to-cell contact sites. Experimental and computer modeling results suggest a role for these channels in electrical coupling between cardiac muscle cells via an ephaptic mechanism working in tandem with gap junctions. This new insight into the mechanism of cardiac electrical excitation could pave the way for novel therapies against cardiac rhythm disturbances.

Stochastic optical reconstruction microscopy-based relative localization analysis (STORM-RLA) for quantitative nanoscale assessment of spatial protein organization

Stochastic optical reconstruction microscopy–based relative localization analysis (STORM-RLA) is a novel method for the high-throughput quantification of spatial protein organization from three-dimensional single-molecule positional data.

The spatial association between proteins is crucial to understanding how they function in biological systems. Colocalization analysis of fluorescence microscopy images is widely used to assess this. However, colocalization analysis performed on two-dimensional images with diffraction-limited resolution merely indicates that the proteins are within 200–300 nm of each other in the xy-plane and within 500–700 nm of each other along the z-axis. Here we demonstrate a novel three-dimensional quantitative analysis applicable to single-molecule positional data: stochastic optical reconstruction microscopy–based relative localization analysis (STORM-RLA). This method offers significant advantages: 1) STORM imaging affords 20-nm resolution in the xy-plane and <50 nm along the z-axis; 2) STORM-RLA provides a quantitative assessment of the frequency and degree of overlap between clusters of colabeled proteins; and 3) STORM-RLA also calculates the precise distances between both overlapping and nonoverlapping clusters in three dimensions. Thus STORM-RLA represents a significant advance in the high-throughput quantitative assessment of the spatial organization of proteins.

Heart Rate and Extracellular Sodium and Potassium Modulation of Gap Junction Mediated Conduction in Guinea Pigs

Background: Recent studies suggested that cardiac conduction in murine hearts with narrow perinexi and 50% reduced connexin43 (Cx43) expression is more sensitive to relatively physiological changes of extracellular potassium ([K⁺]_o) and sodium ([Na⁺]_o).

Purpose: Determine whether similar [K⁺]_o and [Na⁺]_o changes alter conduction velocity (CV) sensitivity to pharmacologic gap junction (GJ) uncoupling in guinea pigs.

Methods: [K⁺]_o and [Na⁺]_o were varied in Langendorff perfused guinea pig ventricles (Solution A: [K⁺]_o = 4.56 and [Na⁺]_o = 153.3 mM. Solution B: [K⁺]_o = 6.95 and [Na⁺]_o = 145.5 mM). Gap junctions were inhibited with carbenoxolone (CBX) (15 and 30 μM). Epicardial CV was quantified by optical mapping. Perinexal width was measured with transmission electron microscopy. Total and phosphorylated Cx43 were evaluated by western blotting.

Results: Solution composition did not alter CV under control conditions or with 15μM CBX. Decreasing the basic cycle length (BCL) of pacing from 300 to 160 ms decreased CV uniformly with both solutions. At 30 μM CBX, a change in solution did not alter CV either longitudinally or transversely at BCL = 300 ms. However, reducing BCL to 160 ms caused CV to decrease more in hearts perfused with Solution B than A. Solution composition did not alter perinexal width, nor did it change total or phosphorylated serine 368 Cx43 expression. These data suggest that the solution dependent CV changes were independent of altered perinexal width or GJ coupling. Action potential duration was always shorter in hearts perfused with Solution B than A, independent of pacing rate and/or CBX concentration.

Conclusions: Increased heart rate and GJ uncoupling can unmask small CV differences caused by changing [K⁺]_o and [Na⁺]_o. These data suggest that modulating extracellular ionic composition may be a novel anti-arrhythmic target in diseases with abnormal GJ coupling, particularly when heart rate cannot be controlled.

Cardiac conduction in isolated hearts of genetically modified mice--Connexin43 and salts

Physiologic variations in perfusate composition have been identified as a new and important modulator of cardiac conduction velocity (CV), particularly when gap junctions (GJ) are reduced. We recently demonstrated in ex vivo hearts that perfusates with low sodium and high potassium preferentially slow ventricular CV in mice genetically engineered to express 50% less of the gap junction protein, connexin43 (Cx43). We also reported the possible role of calcium in modulating CV. In this review we discuss previous murine studies that explored the CV-GJ relationship in isolated mouse heart preparations with approximately 50% reduced Cx43. Studies were grouped according to the type of perfusate utilized, and CV during GJ uncoupling was compared. Studies in Group A preferentially used perfusates with low sodium, high potassium and non-physiologic calcium, and found CV slows and arrhythmias increase in mouse hearts with reduced Cx43. Studies in Group B used solutions with high sodium, low potassium and physiologic calcium, and did not observe CV slowing nor increased arrhythmia risk with loss of Cx3. Studies in Group C used solutions with low sodium, low potassium, physiologic calcium, creatine, taurine, and insulin. CV slowing was not observed, nor was arrhythmia risk increased with loss of Cx43. We suggest that perfusate ion composition may be a major determinant of whether CV slows when Cx43 is reduced. Furthermore, the review of these studies highlights important theoretical developments in the understanding of cardiac conduction and suggests that ionic milieu can conceal electrophysiologic remodeling secondary to reduced Cx43 expression as occurs in many cardiac diseases.

Microdomain effects on transverse cardiac propagation

The effect of gap junctional coupling, sodium ion channel distribution, and extracellular conductivity on transverse conduction in cardiac tissue is explored using a microdomain model that incorporates aspects of the inhomogeneous cellular structure. The propagation velocities found in our model are compared to those in the classic bidomain model and indicate a strong ephaptic microdomain contribution to conduction depending on the parameter regime. We show that ephaptic effects can be quite significant in the junctional spaces between cells, and that the cell activation sequence is modified substantially by these effects. Further, we find that transverse propagation can be maintained by ephaptic effects, even in the absence of gap junctional coupling. The mechanism by which this occurs is found to be cablelike in that the junctional regions act like inverted cables. Our results provide insight into several recent experimental studies that indirectly indicate a mode of action potential propagation that does not rely exclusively on gap junctions.

Arrhythmic substrate, slowed propagation and increased dispersion in conduction direction in the right ventricular outflow tract of murine Scn5a+/- hearts

Aim

To test a hypothesis attributing arrhythmia in Brugada Syndrome to right ventricular (RV) outflow tract (RVOT) conduction abnormalities arising from Na_v1.5 insufficiency and fibrotic change.

Methods

Arrhythmic properties of Langendorff-perfused Scn5a+/− and wild-type mouse hearts were correlated with ventricular effective refractory periods (VERPs), multi-electrode array (MEA) measurements of action potential (AP) conduction velocities and dispersions in conduction direction (CD), Na_v1.5 expression levels, and fibrotic change, as measured at the RVOT and RV. Two-way anova was used to test for both independent and interacting effects of anatomical region and genotype on these parameters.

Results

Scn5a+/− hearts showed greater arrhythmic frequencies during programmed electrical stimulation at the RVOT but not the RV. The Scn5a+/− genotype caused an independent increase of VERP regardless of whether the recording site was the RVOT or RV. Effective AP conduction velocities (CV†s), derived from fitting regression planes to arrays of observed local activation times were reduced in Scn5a+/− hearts and at the RVOT independently. AP conduction velocity magnitudes derived by averaging MEA results from local vector analyses, CV*, were reduced by the Scn5a+/− genotype alone. In contrast, dispersions in conduction direction, were greater in the RVOT than the RV, when the atrioventricular node was used as the pacing site. The observed reductions in Na_v1.5 expression were attributable to Scn5a+/−, whereas increased levels of fibrosis were associated with the RVOT.

Conclusions

The Scn5a+/− RVOT recapitulates clinical findings of increased arrhythmogenicity through reduced CV† reflecting reduced CV* attributable to reduced Na_v1.5 expression and increased CD attributable to fibrosis.

Mechanisms of cardiac conduction: a history of revisions

Cardiac conduction is the process by which electrical excitation spreads through the heart, triggering individual myocytes to contract in synchrony. Defects in conduction disrupt synchronous activation and are associated with life-threatening arrhythmias in many pathologies. Therefore, it is scarcely surprising that this phenomenon continues to be the subject of active scientific inquiry. Here we provide a brief review of how the conceptual understanding of conduction has evolved over the last century and highlight recent, potentially paradigm-shifting developments.